Marin, It's important to be as accurate as possible when creating surfaces. The manner your floor displayed (flashing surfaces) suggest geometric problems with its construction. I raised the non-floor surfaces, recreated the ground surface, replaced the non-floor surfaces, and the floor stopped flashing.
temp.jpg One rule is to avoid using the line tool to create surfaces (unless the surface is a triangle). When some non-coplanar surfaces are joined, their combined surfaces may take on the appearance of a single surface until you attempt to orbit, or render. When you draw coplanar surfaces, pay attention to the colored axis of the surface that is being drawn. When you have two non-coplanar surfaces separated by a space, and if you draw a surface between them (using each of the original surface end points to create the joining surface), unless the original 2 surfaces are perfectly coplanar, the combined three surfaces may resemble, but not be a proper surface.